feat: add gateway-neutral payment package

packages/payment/src/contract.ts

@@ -0,0 +1,38 @@
+import type { PaymentGateway } from "./types.ts";
+
+export interface GatewayContractReport {
+  ok: boolean;
+  issues: string[];
+}
+/** Structural half of the shared adapter suite, usable by third-party adapters in CI. */
+export function verifyGatewayContract(adapter: PaymentGateway): GatewayContractReport {
+  const issues: string[] = [];
+  const capability = (name: string, declared: boolean, present: boolean) => {
+    if (declared !== present)
+      issues.push(
+        `${adapter.id}: capability '${name}' is ${declared ? "declared but not implemented" : "implemented but not declared"}`,
+      );
+  };
+  capability(
+    "authorizeThenCapture",
+    adapter.capabilities.authorizeThenCapture,
+    typeof adapter.capture === "function",
+  );
+  capability(
+    "storedMethods",
+    adapter.capabilities.storedMethods,
+    typeof adapter.listMethods === "function" &&
+      typeof adapter.attachMethod === "function" &&
+      typeof adapter.detachMethod === "function",
+  );
+  capability(
+    "customerVault",
+    adapter.capabilities.customerVault,
+    typeof adapter.createCustomer === "function",
+  );
+  if (!adapter.capabilities.hostedFields && !adapter.capabilities.hostedCheckout)
+    issues.push(`${adapter.id}: a hosted payment surface is required`);
+  if (!adapter.capabilities.webhookSignature)
+    issues.push(`${adapter.id}: signed webhooks are required`);
+  return { ok: issues.length === 0, issues };
+}